1. Field of the Invention
The present invention relates to an information signal recording apparatus and information signal reproduction apparatus, which record or reproduce information on or from a large number of parallel tracks on a recording medium and, more particularly, to tracking control thereof. For example, the present invention relates to tracking control used in an apparatus such as a VTR for reproducing a video signal from oblique tracks recorded and formed on a magnetic tape by a rotary head.
2. Related Background Art
As conventional reproduction tracking methods for home VTRs, the following two methods have been proposed and put into practical applications. That is, one method is a method (CTL method) of recording a vertical sync signal separated from a recording signal upon recording on a special-purpose control track formed along the longitudinal direction of a tape using a stationary magnetic head provided adjacent a portion of a tape path. The other method is a method (4fATF method) of obtaining a tracking error signal by comparing crosstalk components reproduced from two neighboring tracks of a reproduction track upon reproduction by cyclically recording pilot signals having four different relatively low frequencies to be superposed on main signals such as a video signal, an audio signal, and the like on a track for recording the main signals using a rotary head for recording/reproducing the main signals.
However, the CTL method requires a space for the stationary head, and is disadvantageous in consideration of downsizing of a set. On the other hand, the 4fATF method is advantageous for downsizing, but requires a relatively complex circuit arrangement since a large number of analog circuits are required for four different pilot signals. In addition, in the 4fATF method, when the pilot signals are superposed on a digital signal, since these signals occupy a relatively broad frequency band, reliability of the digital signal is undesirably impaired.
On the other hand, in order to record/reproduce a relatively large information amount to meet requirements for high-image quality VTRs and digital VTRs in recent home VTRs, a VTR which divisionally records an image signal for one field on a plurality of tracks has been developed, and a new tracking method used in these VTRs has been examined. More specifically, upon modulation of a digital signal to be recorded, a recording signal is recorded, so that a modulated signal includes a frequency component corresponding to a specific pilot signal, and with this modulation, the pilot signal for tracking control is multiplexed on a digital information signal.
FIG. 1A is a schematic plan view of a drum for a VTR. A rotary drum 40 contacts a tape 41, and has a ch1 head 42 of a (+) azimuth, a ch2 head 43 of a (-) azimuth, a ch3 head 44 of a (+) azimuth, and a ch4 head 45 of a (-) azimuth.
FIG. 1B is a front view of the heads which can be observed upon rotation of the drum for explaining the mounting heights of the heads 42 to 45. The ch1 and ch2 heads and the ch3 and ch4 heads are respectively paired, and are arranged adjacent to each other. These pairs of heads are arranged on the drum 40 to have a 180.degree. angular interval therebetween. As can be seen from FIG. 1B, the ch2 and ch4 heads are respectively offset from the ch1 and ch3 heads by a distance h, and the distance h corresponds to one track pitch on the tape. With this arrangement, two tracks can be simultaneously subjected to recording or reproduction for every half revolution of the drum, thus allowing recording/reproduction of a large amount of information.
A new tracking method will be described below. FIG. 2 is a view showing a recording pattern on the tape 41. Two different pilot signals having frequencies f1 and f2 are used for obtaining a tracking error signal, and are recorded on every other tracks to be superposed on main signals. The generation rotation of the pilot signals requires four tracks to complete one cycle. On a track corresponding to a (+) azimuth of the head, no pilot signal is superposed; on a track corresponding to a (-) azimuth of the head, the pilot signals of the frequencies f1 and f2 are alternately recorded. (1) to (10) in FIG. 2 represent intra-frame numbers of tracks since a signal for one frame is divisionally recorded on 10 tracks.
As described above, in this method, since four tracks are subjected to recording or reproduction during one revolution of the drum, scanning of tracks for one frame requires 2.5 revolutions, and the pilot rotation and frames are synchronized with each other in units of two frames (20 tracks).
FIG. 3 is timing charts showing pilot signals recorded by the heads (ch1 to ch4) to be superposed on main signals, and pilot signals reproduced by the heads upon reproduction. The following explanation will be given with reference to FIG. 3.
FIG. 3, (a) shows a frame signal upon recording or reproduction, FIG. 3, (b) shows a head SW (switch) signal for switching the heads, and FIG. 3, (c) shows the pilot timings of the ch1 and ch3 heads upon recording, at which no pilot signals are superposed. FIG. 3, (d) shows the pilot timings to be recorded by the ch2 and ch4 heads, at which the pilot signals of the frequencies f1 and f2 are alternately recorded. FIG. 3, (e) shows the pilot signals reproduced by the ch1 and ch3 heads in a good reproduction tracking state, and FIG. 3, (f) similarly shows the reproduction timings of pilot components reproduced by the ch2 and ch4 heads.
As can be seen from FIG. 2, in this method, when a head width w of each head is set to be wider than the track pitch, the pilot signals recorded on two neighboring tracks can be reproduced as crosstalk components at the reproduction timings of the ch1 and ch3 heads, and by utilizing the fact that the two crosstalk component amounts become equal to each other in a good tracking state, a tracking error signal (ATF error signal) is obtained.
FIG. 4 is a block diagram showing the circuit arrangement for detecting the ATF error signal upon reproduction.
Referring to FIG. 4, a head SW signal (HSW) 70 is used for switching reproduction signals from the ch1 and ch3 heads in synchronism with the rotation of the drum. An SW circuit 79 switches reproduction signals from the ch1 and ch3 heads in response to the HSW 70. A band-pass filter (BPF) 71 extracts only the frequencies f1 and f2 as those of reproduction pilot signals from a reproduction RF signal, and an amplifier 72 amplifies reproduction pilot signals as the outputs from the BPF 71. A band-pass filter 73 (BPF) extracts only the frequency component f2 from the output from the amplifier 72, and a band-pass filter (BPF) 74 extracts only the frequency component f1 from the output from the amplifier 72. A detecting circuit 75 converts the frequency component f2 as the output from the BPF 73 into a DC level, and a detecting circuit 76 similarly converts the frequency component f1 into a DC level. A differential amplifier 77 receives the outputs from the two detecting circuits. The output from the differential amplifier 77 is supplied to an inverting circuit (amplifier)78. An SW circuit 80 selects one of the output from the differential amplifier 77 and the output from the inverting circuit 78 in response to the HSW 70. A video & audio system reproduced signal processing circuit 81 obtains a reproduction video signal and a reproduction audio signal by processing a reproduction RF signal.
The operation will be described below. As described above, in this system, reproduction pilot signals are included as crosstalk components from two neighboring tracks ((-) azimuth tracks) in reproduction signals from the (+) azimuth ch1 and ch3 heads. Therefore, reproduction signals from only the ch1 and ch3 heads of the four heads are necessary, and the reproduction pilot signals are converted into one system of reproduction signal by an SW circuit 79. Since this reproduction signal is also included in main signals, it is supplied to the video & audio system reproduced signal processing circuit 81, and is also supplied to the BPF 71 for extracting reproduction pilot signals as an ATF circuit. Thereafter, the crosstalk pilot components f1 and f2 are separated and detected, and are compared by the differential amplifier 77, thus obtaining one system of ATF error signal.
In order to cope with the positional switch of tracks corresponding to the frequencies f1 and f2 between the ch1 and ch3 heads, an ATF error signal is obtained by selecting the output from the inverting amplifier 78 by the SW circuit 80 in synchronism with the HSW 70 upon selection of the ch3 head.
The new tracking method has been described. The new tracking method will be referred to as a 2fATF method hereinafter, as needed.
However, the above-mentioned new tracking method poses the following problems. That is, as a first problem, since a tracking error signal is obtained based on only pilot signals reproduced by heads which trace every other tracks (the ch1 and ch3 heads in the above-mentioned method), the ch1 and ch3 heads can be set in a good tracking state. However, other heads (the ch2 and ch4 heads offset by one track pitch from the ch1 and ch3 heads) may often cause an off-track state in accordance with their mounting precision.
As a second problem, the servo characteristics of a tracking control loop change due to a variation in reproduction output level caused by a reproduction level difference between the two pilot signals (frequencies f1 and f2), the performance of a tape to be used, and a difference in characteristic of the heads. In addition, the flutter characteristics in every four-track section of a capstan are impaired. In the conventional 4fATF method, as a countermeasure against these problems, a circuit for performing auto-gain control (AGC) of reproduction pilot signals by detecting pilot levels reproduced from main reproduction tracks has been put into practical application. However, in the new tracking method, since no pilot signals are recorded on tracks mainly traced by the ch1 and ch3 heads for obtaining reproduction pilot signals, the same arrangement as in the 4fATF method cannot be realized.
As a third problem, when information recorded by one head is reproduced by another head which has the same azimuth as that of the former head but is mounted at a different position like in a case wherein information recorded on a track using the ch1 head is reproduced by the ch3 head and vice versa, i.e., when a two-track offset occurs in tracking control, a position where the state of the ATF error signal becomes the same as an on-tracking state (back lock position) is present, and the presence of the back lock position disturbs satisfactory tracking control. In other words, a long period of time is required until the back lock position returns to a normal lock position, and an image is disturbed during this interval.
Furthermore, as a fourth problem, in the new tracking method, the pilot section requires four tracks to complete one cycle. For example, when a signal for one frame is divisionally recorded on 10 tracks, the pilot section and the frame section coincide with each other in units of two frames (20 tracks). In addition, when pilot signals are discontinuous in a joint recording portion, tracking control and an image are disturbed. As described above, in order to smoothly perform joint recording operations, a long preparation time is required.